The use of drives and transmissions in mechanical and electromechanical systems is quite common for such applications as motion control, electronics, machine tools, printing machines, robotics and aerospace. In many situations involving actuators and electromechanical systems, a high gear reduction ratio is useful.
Typically, high gear reduction ratios are achieved by having multiple gear stages. In applications where size and weight is of less concern, having multiple gear stages is an appropriate method to achieve high gear reduction ratios. However, in many applications, size and weight are of great concern.
Further, in some applications, such as robotics or actuators, joints that permit the device to move typically have a fixed reduction ratio transmission system, which may not provide an efficient force/speed ratio for the wide range of operating conditions of the device. This type of design, where a fixed ratio transmission system is provided, often leads to inefficient use of energy sources, which is a particular issue in applications that rely on battery power, such as mobile applications. These inefficiencies limit the practical use of such devices in many applications. Improving the efficiency of energy conversion generally leads to longer operation time for a given a battery source. Greater efficiency may also permit smaller motors to be utilized, which may enable more compact designs to be utilize for both mobile and stationary mounted applications.
Another problem for electromechanical actuator systems is that they are often designed for worst case load or speed conditions, making them big and bulky. Designing a device that can quickly adapt to changing load conditions and operate with high efficiency would also be desirable.
Transmission systems with large gear reduction ratios in a single stage are available commercially. An example of a commercially available device that provides a high gear reduction ratio in a single stage is a harmonic drive. A basic harmonic drive includes a fixed circular spline with teeth formed on its inside surface, a flex spline made of flexible material with teeth formed on its outside surface that engage with the teeth of the circular spine, and an elliptical wave generator that is tightly fit within the flex spline. The flex spline has fewer teeth than the circular spline. When the wave generator is rotated by an input shaft, it deforms the flex spline, which is connected to an output shaft, into an elliptical shape that causes the teeth of the flex spine to engage the teeth of the circular spline. Because the flex spline has fewer teeth than the circular spline, the rotation causes different teeth of the flex spline to engage the circular spline resulting in a slight backward rotation of the flex spline with respect to the circular spline. The teeth of the circular and flex splines typically feature fine gearing that requires the use of high-precision machined parts, which makes them expensive. Because harmonic drives also rely on the flexing of component materials, they often experience high rates of wear and may result in the use of heavier and larger parts to compensate for wear or require frequent replacement of worn out drives. As a result, harmonic drives are often relatively large, heavy and expensive to operate.
In many applications, there is also a need to monitor and sense how hard the actuator may be pushing against a load. Typical solutions for monitoring include adding some type of force sensor, which adds to the cost and complexity of the device. It may be advantageous to use the actuator itself in a back-drivable mode to act as a sensor such that the current through the actuator monitored and may be related to the force applied by the actuator. However, for a drive system to be back-drivable, it needs to be efficient without significant friction losses encountered when driven backwards. Typical on-market systems do not have the desired characteristics to utilize the actuator as a sensor.
Thus there is a need for single stage, efficient, light-weight, compact, low-cost drive systems that achieve high gear ratio reduction.